Question

A compound microscope has an eye piece of focal length $10cm$ and an objective of focal length $4cm.$ Calculate the magnification. If an object kept at a distance of $5cm$ from the objective so that image is formed at the least distance vision $(20cm)$

• A. 12
• B. 11
• C. 10
• D. 13

Answer 1

Answer: (A)

For objective lens $=\frac{1}{f}=\frac{1}{v_0}-\frac{1}{u_o}$
$\frac{1}{v_0}=\frac{1}{f_0}-\frac{1}{u_0}=\frac{1}{4}+\frac{1}{-5}$
$\frac{1}{v_o}=\frac{1}{20}\Rightarrow v_0=20cm$
Now, $m=\left|\frac{v_0}{u_0}\right|\left(1+\frac{D}{f_e}\right)$
$m=\frac{20}{5}\left[1+\frac{20}{10}\right]=4\left(\frac{30}{10}\right)=12$